The present invention relates to multimetal oxide materials essentially of the formula
Mo12BiaX1bFecX2dX3eOyxe2x80x83xe2x80x83(I),
where
X1 is Co and/or Ni, preferably Co,
X2 is Si and/or Al, preferably Si,
X3 is an alkali metal, preferably K, Na, Cs and/or Rb, in particular K,
0.3xe2x89xa6axe2x89xa61,
2xe2x89xa6bxe2x89xa610,
0.5xe2x89xa6cxe2x89xa610,
0xe2x89xa6dxe2x89xa610,
0xe2x89xa6exe2x89xa60.5 and
y is the absolute value of the number which is obtained, assuming charge neutrality, from the valences and the stoichiometric coefficients of the other elements, the crystalline fractions containing, in addition to xcex2-(Co or Ni)MoO4 as the main component, Fe2(MoO4)3 as a secondary component and no MoO3. The present invention furthermore relates to the preparation of the multimetal oxide materials, moldings produced therefrom and the use of the multimetal oxide materials and of the moldings.
The expression xe2x80x9cessentiallyxe2x80x9d preferably means a content of at least 95, particularly preferably at least 99, % by weight. In particular, no other components are present.
Similar multimetal oxide materials are disclosed in EP-A 0 000 835. According to this publication, these multimetal oxide materials are formed by preforming, in the absence of the other constituents, the bismuth-molybdenum mixed oxide forming the catalytic key phase of these multimetal oxide materials, mixing said mixed oxides with sources of the other constituents of the multimetal oxide materials after its pre-formation and calcining the mixture after drying.
EP-B 0 575 897 discloses multimetal oxide materials which have three-demensional regions having the chemical composition Bi2W2O9 with a maximum diameter of from 1 to 25 xcexcm. These regions are delimited from their local environment owing to their chemical composition. The known materials are formed by means of a process in which first a finely divided powder comprising calcined mixed oxide is initially taken and the sources of the other components of the multimetal oxide materials are then added.
Both above-mentioned multimetal oxide materials are used for the gas-phase catalytic oxidation of organic compounds. Apart from their time-consuming and labor-intensive preparation, the known catalytic multimetal oxide materials are furthermore not completely satisfactory with respect to their selectivity.
EP-A-0 493 274 and DE-A-27 41 132 as well as DE-A-31 14 709 describe the preparation of multimetal oxide materials by precipitation, a mixed metal salt solution of the starting metals being precipitated with ammonium molybdate solution. The activity and selectivity of the catalysts obtained are not adequate for all applications.
It is an object of the present invention to provide multimetal oxide materials and a process for their preparation which do not have the above-mentioned disadvantages. In particular, the preparation is to be simplified; moreover, the multimetal oxide materials are to be improved with respect to their activity and selectivity in the gas-phase oxidation and are to permit a higher space-time yield.
This object is achieved by the multimetal oxide materials stated at the outset. The determination of the components is carried out by means of X-ray diffractometry.
Fe2(MoO4)3 is present as a chiefly occurring secondary component. Preferably, Bi2Mo3O12, X16Mo12Fe4Bi1.5Ox or a mixture thereof is present as an additional component in the crystalline fractions, x having the meaning stated for y.
Preferably, no Bi3FeMo2O12 is present in the crystalline fractions.
X1 may be Co and/or Ni. Mixtures having a predominant proportion of Co are preferred; in particular, X1 is Co.
The present invention also relates to a process for the preparation of such multimetal oxide materials, in which
a) a first aqueous solution A of the constituents Bi, Fe and X1 is prepared,
b) a second aqueous solution B of the constituents Mo and X3 is prepared,
c) the solutions A and B are mixed, a solution or suspension of the constituent X2 being mixed with the solution A, B or a mixture thereof,
d) the mixture or precipitated product obtained in c) is dried and
e) the product obtained in d) is calcined.
Preferably, the aqueous solutions A and B are mixed and the mixture thereof is mixed with a solution or suspension of the constituent X2.
The calcination is preferably effected at from 450 to 490xc2x0 C., in particular from 460 to 480xc2x0 C., especially from 465 to 477xc2x0 C.
The drying is preferably effected by means of spray drying.
It is preferable according to the invention if the multimetal oxide materials of the formula I are prepared by means of a one-pot process. Here, a first aqueous solution having the constituents Bi, Fe and X1 is first prepared. The preparation of the first aqueous solution is effected from suitable water-soluble starting compounds of the constituents Bi, Fe and X1. Such water-soluble compounds of said constituents are sulfates, halides, acetates, formates, nitrates, carbonates, etc. Among these water-soluble starting compounds, the nitrates are preferred for preparation of the first aqueous solution. To prepare the first aqueous solution, it may be necessary to dissolve the starting compounds at elevated temperatures, in particular at from 50 to 70xc2x0 C.
A second aqueous solution B of the constituents Mo and X3 is similarly prepared. In particular, ammonium molybdate is used for this purpose. The alkali metal X3 is preferably potassium, which is used as KOH. This solution, too, can be prepared at from 50 to 70xc2x0 C. The two solutions are then mixed, for example by pumping solution A into solution B. The constituent X2 may be contained in solution A, solution B or the mixture or may be added to it in the form of a solution or suspension. Preferably, a silica sol is added to the mixture if X2 is silicon in the mixture.
Mixing of the solutions results in precipitation of the metals.
If the catalytic multimetal oxide material is to have pores, it is preferable to use ammonium salts or nitrates as the starting compounds as feedstocks so that a pore-containing containing catalyst forms.
If no ammonium salts or nitrates are used, it is also possible to add ammonium nitrate, as a pore former, to one of the two aqueous solutions or to both aqueous solutions. If required, a finely divided suspension of alumina particles and/or silica particles can also be added to one or both abovementioned aqueous solutions. The alumina or the silica acts as an inert diluent.
After precipitation is complete, the precipitate is separated from the remainder of the solvent. For this purpose, it can be dried by evaporation. However, the separation is preferably effected by means of spray-drying. Spray-drying can be carried out by the cocurrent or countercurrent method. During the spray-drying, the temperature at the spray tower inlet is preferably about 400xc2x0 C. and the temperature at the spray tower outlet from about 120 to 140xc2x0 C. The spray-drying gives particles of the novel multimetal oxide material having a diameter of from about 1 to 100 xcexcm. The powder obtained after the spray-drying is further processed depending on the desired catalyst formed. For the preparation of annular catalysts, the powder obtained is pelleted with the use of not more than 4% by weight of graphite (BET surface area 5-15 m2/g, mean particle diameter  less than 30 xcexcm) as a pelleting aid and is then decomposed at preferably from 150 to 300xc2x0 C. for oxide formation. The resulting oxide material is then calcined at the abovementioned temperatures. The process step involving decomposition and that involving calcination can be carried out in an inert gas atmosphere, in air, under oxygen, nitrogen, etc. The calcination and the decomposition can be contiguous process steps. For the preparation of catalysts in chip form, the powder obtained after the spray-drying is kneaded, then dried and then coarsely milled. It is then decomposed and calcined as described and subsequently converted into chips and separated by sieving.
For the preparation of coated catalysts, the powder obtained after the spray-drying is kneaded, dried and coarsely milled and then decomposed as described. After the decomposition, milling to a defined particle size is effected, the catalyst carrier is coated and only thereafter is calcination effected.
The multimetal oxide material of the formula I Mo12BiaX1bFecX2dX3eOy, obtained by means of the novel process, contains Co and/or Ni, but preferably Co, as X1. X2 is Si and/or Al, preferably Si. X3 comprises alkali metal elements Li, Na, K, Cs and/or Rb, preferably K, Na and/or Cs, particularly preferably K; the value of the variable a is from 0.3 to 1. Preferably, 0.4xe2x89xa6axe2x89xa60.1, in particular 0.4xe2x89xa6axe2x89xa60.95. The value of the variable b is from 2 to 10, preferably 4xe2x89xa6bxe2x89xa68, very particularly 6xe2x89xa6bxe2x89xa68; the value of the variable c is from 0.5 to 10, preferably from 1 to 5, in particular from 2 to 4. The value of the variable e is, according to the invention, from 0 to 0.5, in particular  greater than 0. Preferably, 0.01xe2x89xa6exe2x89xa60.5, very particularly preferably 0.05xe2x89xa6exe2x89xa60.2
The value for O (oxygen) is obtained from the absolute value of the sum of the valences and the stoichiometric coefficients of the other elements, i.e. of the cations. In the novel process, a multimetal oxide material in which the Co/Ni ratio is at least 2:1, preferably at least 3:1, particularly preferably at least 4:1, is preferably prepared. Very particularly preferably, only Co is present. The multimetal oxide material obtainable by means of the novel process is distinguished only by the overall composition of its components which is described above but also by a typical phase composition, as demonstrated in the Examples with reference to X-ray diffraction patterns.
In the case of particularly preferred novel multimetal oxide materials, the value for 1.5 (a+c)+b is from 11 to 14, preferably from 11.5 to 13, particularly preferably from 11.8 to 12.5. The limiting values are included in the stated ranges.
According to the invention, the novel multimetal oxide material is used for the preparation of moldings, in particular of catalyst moldings. For this purpose, it is used in the form of a powder. The shaping itself can be used before or after the calcination. Possible moldings are unsupported catalysts or coated catalysts. Coated catalysts prepared from the novel multimetal oxide material or annular unsupported catalysts, which can be obtained by pelleting or extrusion, are preferred. In addition, the catalyst may be present as chips.
In addition to the multimetal oxides, the catalyst may also contain further bismuth-containing oxides, such as Bi2W2O9 or Bi2Mo3O12. The oxides may be mixed in the form of a powder and processed to give catalysts.
The novel multimetal oxide materials are suitable for the selective gas-phase oxidation of organic compounds, in particular alkenes, such as propene. In particular, they are suitable for the preparation of xcex1,xcex2-unsaturated aldehydes and/or carboxylic acids from alkenes, alkanes, alkanones or alkenals. Particularly preferably, the novel multimetal oxide materials are used for the preparation of acrolein and acrylic acid from propene and methacrolein, respectively, and methacrylic acid from isobutene.